1. Field of the Invention
The present invention relates to a picture signal coding/decoding method and a picture signal coding/decoding device, and more particularly, is suitably applied to the case of transmitting picture signals upon, such as, component separate coding by Wavelet transformation using Haar function.
2. Description of the Related Art
Heretofore, in a picture signal transmission system to transmit picture signals to a remote location, e.g., a television conference system used, and a device to record and reproduce picture signals on a video tape recorder and video disc recorder, by effectively coding a significant information by utilizing the correlation of digital picture signals in order to use the transmitting and recording medium effectively, an amount of transmitting information and recording information are decreased and transmitting efficiency and recording efficiency are improved.
In known coding methods to utilize the correlation of picture signals, a method for quantizing and transmitting after separating picture signals into multiple components, such as predictive coding method, orthogonal transform coding system, such as DCT (Discrete Cosine Transform), sub-band coding system and Wavelet transform system is used.
The predictive coding method is easily installed and suitable for coding of fairly low compressibility. However, it has a weak point that worsening of picture quality is apt to be detected when the compressibility is increased. Also, the orthogonal transform coding system, such as DCT, is widely used since high picture quality can be obtained relatively easily with high compressibility. However, there are weak points that noticeable distortion occurs on the border of blocks and the trouble which occurs because the degree of distortion is different in each block is noticeable. Furthermore, in the orthogonal transform coding system with DCT, the large amount of calculation becomes an obstacle for the hardware.
Furthermore, the component separate coding, such as the sub-band coding system and Wavelet transform system, performs quantization after separating picture signal into multiple components, and relatively high compressibility can be obtained and there are few cases where such peculiar distortion occurs. The installation process differs according to the method, but there is the method, such as, Wavelet transform using Haar function, which can be easily constructed.
Wavelet transform method performs component separation recurrently on low frequency components and performs coding on the resultant components i.e., in conformity to its characteristics in each coefficient. And there are filters for separation and reconstruction as filters to use, for example, Haar function, as shown in FIGS. 1A to 1D combining one sample each of delay circuit, multiplier and adder, and there are other various kinds of filters which are properly used according to the purpose.
Furthermore, in case of applying the component separate coding, such as Wavelet transform to the coding of motion picture signals, such as an ordinary television signal in which one frame is constructed by two fields, the intra field separate coding method has higher efficiency for the picture pattern with vigorous motion whereas the intra frame coding has higher efficiency on the still picture pattern or almost stopped picture pattern.
Accordingly, in the component separate coding of such as DCT and Wavelet transform using Haar function, the statistical characteristic of picture signal is calculated and the method to shift the intra field coding and the intra frame coding is applied referring to the calculation result. For example, a picture coding device is constructed as shown in FIG. 2.
More specifically, in this picture coding device, input signal S1 is firstly inputted to a framing circuit 1 and frame picture signal S2 in which the first field lines and second field lines are scanned alternatively is formed. This frame picture signal S2 is inputted to a judging circuit 2 which judges either the intra field coding or intra frame coding is suitable. The judging circuit 2 calculates, such as, the correlation of vertical direction of the frame picture signal S2 and in the case where correlation is strong, intra frame coding is selected, and if correlation is weak, blocking shifting information S3 to select the intra field coding is outputted to a transforming block construction circuit 3.
On the other hand, the frame picture signal S2 is inputted to a memory 4 which delays for a time required for processing of the judging circuit 2 and is then inputted to a transforming block construction circuit 3 from memory 4 once it is delayed for the prescribed time. A transforming block construction circuit 3 forms either picture block data for transformation using picture elements in the field only or picture block data for transformation using picture elements in the frame.
This picture block data for transformation S4 is inputted to the transform circuit 5 and transformed therein to become coefficient S5. This transform circuit 5, as shown in FIG. 3, is composed of 2-dimensional Wavelet transform circuit, divides picture block data for transformation S4 into the high frequency components and the low frequency components and by repeating the down sampling to thin out the resultant samples of both components on every other sample on the low frequency components, performs Wavelet transform utilizing, for example, Haar function as shown in FIG. 4.
Coefficient S5 to be outputted from the transform circuit 5 is inputted to a quantizer 6. At quantizer 6, it is quantized according to quantization step information S6 to show the quantum efficiency to be determined for smoothing the amount of information generated based on the amount of storage of a buffer memory 8 installed at the lower stage. The resultant quantization coefficient S7 is inputted to a variable length coding circuit 7.
At the variable length coding circuit 7, variable length coding, such as Huffman coding and zero run-length coding are combined, is applied and the resultant variable length coding data S8 is inputted to a multiplexer 9 via the buffer memory 8 for smoothing the amount of information generated. And here, it is multiplexed with quantization step size information S6 and blocking shifting information S3 and outputted as output data S9 of the picture coding device.
Output data S9 of the picture coding device is inputted to the picture decoder as shown in FIG. 5 as input data S11 and is decoded. More specifically, in the picture decoder the input data S11 is inputted to a shunt circuit 11. The shunt circuit 11 separates quantization coefficient S12 which is variable length coded from the input data S11, and quantization step size information S13 and blocking shifting information S14 and outputs them.
The quantization coefficient S12 which is variable length coded is inputted to a variable length decoder 13 via a buffer memory 12 and variable length decoded to become quantization coefficient S15 and is inputted to a dequantizer 14. The dequantizer 14 dequantizes the quantization coefficient S15 according to the quantization step size information S13 and the resultant dequantization coefficient S16 is inputted to an inverse transform circuit 15 and inverse transformed to become restored picture signal S17.
The inverse transform circuit 15 is composed of two-dimensional Wavelet inverse transform circuit as shown in FIG. 6, and by repeating data reconstruction operation on high frequency elements successively after performing up-sampling to insert zero between each sample of low frequency components and high frequency components, performs, for example, inverse transform as shown in FIG. 4 and restores picture data S17.
The picture data S17 restored at the inverse transform circuit 15 is returned to framing picture signal S18 according to block shifting information S14 at the framing circuit 16 and is returned to field scanning picture signal S19 at a field circuit 17 from which it is outputted as restored picture signal S19 which is the output of the picture decoding device.
However, in the case of picture coding and picture decoding as described above, the conventional apparatus is effective on picture patterns on both extremities of vigorous motion picture patterns and almost still picture patterns. But there has been a problem that effective compressibility and visual picture quality could not necessarily be improved for intermediate picture patterns containing slow movement.
Moreover, since there are cases where visible distortion occurs on the border of the intra-frame coded block and the intra-field coded block, the conventional apparatus has not been sufficient as a coding method for coding picture signals of high picture quality. Furthermore, in the case of simple decoding in utilizing the low frequency coefficients, it was unavoidable that picture decoding device became complicated and large sized since the blocking format of the low frequency coefficients are different in the intra-field coding and intra-frame coding.
Furthermore, in the picture coding device, the judging circuit to judge shifting of memory and block to be used at the framing circuit is large in scale and in the case where the device scale of the coding side is strictly restricted, it has been difficult to obtain small sized device allowing for simple decoding, such as a monolithic camera video tape recorder.